Its the same C# in both. What's the problem?
What have you tried so far?

Did you build the desktop application using WPF? That really should be your starting point if you built the desktop application in WinForms. WinForms is yesterday. WPF is today and tomorrow.

Have you worked through any intro Win8-metro tutorials or books to help you bridge the gap?? MSDN is full of resources on the topic. You probably want to start with a Metro "hello world" and go from there. If you can already build in C# it won't take long to come up to speed.

The only real difference is the client you'll have to use. With the RT SDK, you'll use the HttpClient instead of WebClient. And RT doesn't support doing synchronous calls, so you'll have to either use callbacks, or use the async and await keywords. There's an example of this in the link I provided.

But the CRM api will be the same. First, let me ask: are you using the ODATA endpoints, or the Organization Service endpoint? If all you're doing is retrieving, the ODATA endpoints will be simpler.

edit: sidenote, yay, I finally get to put my year of CRM 2011 experience to use on this forum.